Borders divide people, states, institutions, and traditions. In her nuanced book, Annette Idler focuses instead on what borders connect: communities, illicit markets, and violent organizations.

We might expect the numerous violent nonstate groups along Colombia’s borders with Venezuela and Ecuador to be competitors—for ideological reasons in the case of Colombian insurgent groups and counterinsurgents, or for economic ones in the case of lucrative illicit markets. But Idler provides a conceptual framework for the numerous ways these groups cooperate, narrowly or broadly. She then draws out implications of these arrangements for civilians’ security in the borderlands of Colombia/Venezuela and Colombia/Ecuador. In this way, the book speaks to the literatures on armed group alliances, criminal conflict, and rebel governance.

Idler develops multifaceted theories to answer three questions: What are violent nonstate actors’ relationships and what explains them? What impact do these relationships have on citizen security? And what is the effect of the border on these violent nonstate groups’ relationships and civilians? Chapter 2 classifies violent nonstate actors’ relationships into three “clusters:” enmity, rivalry, and friendship. Within each cluster are more specific arrangements, such as combat and armed disputes in enmity, spot sales and tactical alliances in rivalry, and supply chain relationships and pacific coexistence in friendship (p. 40). Idler associates the clusters with distrust-reducing mechanisms that allow groups to work together, such as personal connections, mutual interests, and shared values. After providing an overview of the borderland settings in chapter 3, readers curious about the fieldwork and the research design should detour to the epilogue and appendix A before moving on to the chapters that provide more in-depth overviews of the three clusters.

Chapters 4–6 describe each cluster and civilians’ experiences living under each. Each of the clusters has implications for what Idler calls “citizen security,” which encompasses several dimensions, such as social fabric and a reciprocal relationship with the provider of governance and the citizen. Security is both “objective” and “subjective,” with the latter going beyond measures of observable violent events to include psychological effects of violence such as fear and coping.

Chapter 4 focuses on the enmity cluster and how the interactions that comprise it—combat and armed disputes—affect the citizens who live in the contested areas. Citizens can remain safe if they avoid armed disputes in cities or combat sites in the rural areas. Chapter 5 considers how the different arrangements under rivalry affect citizens. Surprisingly, in this mode in which armed groups cooperate, albeit guardedly, citizen security is worse than when enmity or friendship prevails. Here, uncertainty pervades communities, and selective violence can affect people who are unclear about who is in charge. Chapter 6 looks at areas of “friendship,” where civilians have the best chance at something resembling security, even if it is provided by violent nonstate groups. Here, under strategic alliances, pacific coexistence, and preponderance, these violent groups can develop governance relationships with civilians living in the affected areas. Idler characterizes this as shadow governance and shadow citizenship.

In Chapter 7, Idler considers the effect of the border on these relationships among nonstate groups and civilians. She identifies four ways in which the border functions: as facilitator, deterrent, magnet, and disguise (p. 252). The first two relate to the border per se and its transnational character, and the second two relate to the distance to the center. Borders facilitate illicit trade and deter interventions from the state. They also serve as a magnet for illicit groups and disguise the behavior of these groups from the central political authorities. Chapter 8 raises areas for new research after covering some examples of borderlands in other regions. Idler suggests considering whether the nature of borders matters, as well as the type of organized crime present, exploring how nonstate order and citizen security move beyond the war and peace binary, and further examining how nonstate actors, civilians, and the state interact.

Key contributions of the book are that it draws attention to the relationships among violent nonstate groups and provides detailed accounts of borderland residents’ lives. The civil war literature has conceived of relationships between states and nonstate groups, and between armed actors and civilians. Comparatively little has been said about the arrangements among nonstate armed actors, and the limited research on this subject tends to focus on where and when alliances emerge. This book provides many interesting examples of how these groups interact between these poles. In addition, the work contributes to the rebel governance literature by uncovering new circumstances under which governance relationships emerge that do not necessarily depend on the predominance of one group. The fieldwork that Idler conducted is careful, and her commitment to telling the stories of often-marginalized communities is a contribution in itself.

These strengths would be even more forceful if the book resolved two outstanding issues. One question is how the clusters and the arrangements within them relate to one another. To make an obvious pun, the borders between these concepts and their empirical instances remain fuzzy. I was not sure how to classify an alliance as tactical or strategic or how to identify peaceful coexistence compared to strategic alliances. Idler writes that the clusters constitute order, but it was not always clear how durable each instance was. If these are empirically and conceptually important relationships, it would be helpful to have the tools to tell them apart from one another, to identify where and when they exist elsewhere, and when they transform into a new type of relationship.

A second question relates to the importance of the setting. The book claims that borderlands are special both because of their transnational character and their distance from seats of political power. Yet much of what Idler describes, such as enmity between groups, strategic alliances, and shadow governance, is found in areas of Colombia far from the borders, as she notes at various points. As a result, the ways that the borderlands differ from the interior are often elided. To me, two distinctions stand out. First, illicit commodities tend to have huge price jumps at the border. How does this change the nature of the relationships between the nonstate armed groups at the borders compared to how they relate in the interior, at earlier points in the supply chain? Should we expect a higher prevalence of tactical and strategic alliances in borderlands than in the interior? Second, borders are also unique because they are points where neighboring state authorities could also cooperate or compete. Under what conditions should we expect them to do so? Does enmity, rivalry, and friendship also describe the relationships of cross-border state authorities? And how do these relationships among state actors influence violent nonstate groups and civilians?

Idler provides a window into the lives of civilians who negotiate incredibly difficult situations, into the diversity of relationships among violent nonstate groups, and into regions typically left on the margins. In the process, her work raises new and important questions about civil wars and criminal conflict.